Various methods are known in the art for growing crystalline bodies from a melt. For example, using the "EFG" process (also known as the "edge-defined, film-fed growth" process) disclosed in U.S. Pat. No. 3,591,348, issued to Harold E. LaBelle, Jr., it is possible to grow crystalline bodies of silicon or other materials in diverse shapes of controlled dimensions by means of so called capillary die members which employ capillary action for replenishing the melt consumed by crystal growth. Also, by introducing suitable conductivity type-determining impurities or dopants, to the melt, e.g., boron, it is possible to produce crystalline bodies by the aforesaid EFG process which have a P or N type conductivity and a predetermined resistivity. For silicon solar cells, it is preferred that the resistivity of such regions be held to less than about 100 ohm-cm and for best conversion efficiency between about 0.001 to about 10 ohm-cm.
P-N junctions are formed in such materials by introducing a selected impurity or dopant into the crystalline body, e.g., phosphine is introduced to form an N-type layer in boron-doped P-type silicon, whereby a P-N junction is created. Also in order to improve the efficiency of collecting the photoelectrically produced carriers in such solar cells, the depth of the P-N junction from the surface which is to serve as the radiation receiving surface is made small, preferably on the order of 0.5 micron.
Another process for growing crystalline bodies with controlled cross-sectional shapes is disclosed by U.S. Pat. No. 4,000,030 issued to Ciszek. In this patent, the method involves the use of a submerged projection extending above the level of the melt, with the crystal's growth occurring from a melt meniscus formed over the upper end of the projection.
As disclosed in U.S. Pat. No. 4,036,666 issued to Mlavsky it has been found that ribbons can be produced by growing a substantially monocrystalline tube and then slicing the tube lengthwise. The ribbons produced do not have the concentration of surface defects adjacent their edges as characterizes ribbons grown directly from the melt by EFG. The foregoing discovery and a succession of other discoveries has led to the growth of regularly shaped octagons or nonagons (i.e., hollow bodies with cross-sectional configurations in the shape of eight or nine sided polygons) as a preferred configuration. The polygons are later cut at their corners to produce flat ribbons.
In the formation of solar cells from P-type EFG-grown silicon ribbon, the ribbon is provided by growing it from a boron-doped, semiconductor grade silicon melt under an inert atmosphere of argon gas using the EFG process. With P-type ribbon, solar cell formation is typically achieved by introducing the ribbons into a diffusion furnace where they are exposed to phosphorous oxychloride under conditions conductive to diffusion of phosphorous into the surface of the ribbons so as to form a continuous N+ layer around the entire cross-section of the ribbons. Thereafter a silicon nitride or other anti-reflection coating is deposited on the front side of the ribbon substrate and electrodes are applied to both the front and rear sides of the ribbons, according to known techniques (see U.S. Pat. Nos. 4,451,969; 4,609,565; and 4,557,037).
In the typical diffusion type junction-forming operation, diffusion occurs on all surfaces of the substrate, including the sides and edges. Consequently, the cell edges have to be trimmed to eliminate a low resistance current path ("short circuit") between the front and rear sides of the solar cell and thereby conductivity isolate the back of the cell from the front. Trimming may be accomplished by mechanically sawing off edge portions of the cells. More recently, lasers have been used to cut off the edges of the cells.
These techniques for isolating the backside of the cell are effective. However, diffusion and edge trimming are also costly and wasteful. Approximately 20-30% of the total cost of making a solar cell is incurred during these steps. Diffusion and edge trimming require a series of labor-intensive and materials-intensive operations, both of which contribute to yield losses. In addition to these expenses, the loss of the edges reduces the power producing area of the cell by nearly 5% and silicon trimmed from the cell is discarded as waste, which must be disposed of within the guidelines established by the government pollution control authorities.
Moreover, it is recognized by persons skilled in the art that widespread use of photovoltaic solar cells is dependent upon the development of fabrication techniques capable of producing reliable solar cells with a conversion efficiency of 12% or higher at a relatively low cost. The cost and saleability of solar cells, like other semiconductor devices, depends on (1) the cost of the starting materials, (2) the cost of converting the starting materials into the finished product, (3) the cost of disposing of waste materials, (4) the overall output of the cells, and (5) the yield of acceptable solar cells.